Method for operating an oscillator and an electric circuit arrangement and phase locked loop

ABSTRACT

The invention relates to a method for operating a voltage-controlled oscillator ( 3 ). The invention also relates to an electric circuit arrangement and a phase-locked loop. The invention is characterised in that a certain additional voltage (U 2 ) is added to the variable control voltage (U 4 ) in an operating phase when required. Said control voltage is supplied to the oscillator ( 3 ) by means of a capacitor ( 2 ). The certain additional voltage (U 2 ) is mounted on the capacitor ( 2 ) in a loading phase.

[0001] The invention relates to a method for operating a voltage controlled oscillator, in particular in a portable communications appliance, with the oscillator being supplied with a variable control voltage which is taken from an operating voltage, preferably from a constant voltage source.

[0002] The invention furthermore relates to an electrical circuit arrangement for controlling a voltage controlled oscillator, with the oscillator having at least one control input for a variable control voltage, and at least one input for an operating voltage. The invention furthermore relates to a phase locked loop having at least one detector which produces a variable control voltage, having a filter arrangement and having an oscillator, with the oscillator having at least one control input for the variable control voltage, and at least one input for an operating voltage.

[0003] Voltage controlled oscillators are used in communications technology appliances in order to produce radio-frequency sinusoidal voltages. In these oscillators, the output frequency can be varied or tuned within a specific range via a control input. In this case, the magnitude of this tunable frequency range, which is also referred to as the pull-in range, is defined by minimum and maximum voltage which can be supplied to the control input. In this case, the maximum voltage is governed by the available operating voltage.

[0004] Two requirements often conflict with one another during operation of modern communications appliances. These communications appliances frequently require the oscillator to have a wide pull-in range, but at the same time operate with low operating voltages, for example because they are battery-powered. Oscillators which cover a wide pull-in range with only a low control voltage can admittedly be produced, but often suffer from high sensitivity and poor phase noise characteristics. This is often further exacerbated by the use of such oscillators in a phase locked loop.

[0005] Until now, in order to solve this problem, the operating voltage of the overall appliance has, for example, been increased by using DC/DC converters or charge parts. However, this solution is expensive, and there is a risk of spectral impurities due to AF interference (AF=audio frequency). Furthermore, a number of oscillators are used, each of which covers only a portion of the desired frequency range. However, this variant now results in enormous costs. Switched oscillators are used in another proposal, in which a sudden frequency change can be produced by switching the resonator circuit, generally by means of PIN diodes. However, in this case, the design and configuration of such an oscillator are problematic.

[0006] The object of the invention is thus to provide a method which allows the pull-in range of an oscillator to be extended in a simple and disturbance-free manner. A further object of the invention is to provide an electrical circuit arrangement and a phase locked loop for carrying out the method.

[0007] The object of the invention is achieved by the respective independent claims.

[0008] Accordingly, the inventor proposes the further development of a method for operating a voltage controlled oscillator, in particular in a portable communications appliance, with the oscillator being supplied with a variable control voltage, which is taken from an operating voltage, preferably from a constant voltage source, such that the variable control voltage is supplied via a capacitor to the oscillator, and a specific additional voltage is added to the variable control voltage when required in an operating phase, with the specific additional voltage being inclined to the capacitor in a preparation phase. The method for operating the voltage controlled oscillator is thus broken down into a preparation phase and an operating phase.

[0009] The capacitor is advantageously charged during the preparation phase with the aid of a charging voltage, which is applied to the capacitor. This charging voltage may now be equal to the operating voltage, so that, at the end of the preparation phase, the capacitor is at a voltage (additional voltage), which is equal to the operating voltage. In the operating phase, this additional voltage may be added to the variable control voltage, as a result of which the input voltage range of the oscillator is increased by the magnitude of the additional voltage, and the pull-in range of the oscillator is in consequence also widened.

[0010] One advantageous further development of the method according to the invention provides for the charging time to be controlled by interrupting the charging voltage which is applied to the capacitor. The charging process of the capacitor in the preparation phase may be interrupted, for example, by means of a control apparatus, which opens a switch after a predetermined charging time, and thus interrupts the charging of the capacitor. The additional voltage which is now present across the capacitor is less than the charging voltage, or less than the operating voltage. The magnitude of the additional voltage across the capacitor is thus dependent on the duration of its charging time. This further development means that the control voltage can be continuously variably increased, so that the pull-in range of the oscillator can also be tuned over a wide frequency range.

[0011] A further embodiment of the method according to the invention provides for an additional voltage to be produced, which is in the opposite sense to the charging voltage. This makes it possible for negative additional voltages to be present across the capacitor, and henceforth even greater variability to be achieved.

[0012] In another embodiment of the method according to the invention, when a low control voltage is desired across the oscillator rather than a high control voltage, the capacitor is discharged to a voltage 0 during the preparation phase. The control voltage, unchanged by the capacitor, is thus produced across the oscillator in the operating phase.

[0013] Furthermore, the capacitor is neither charged nor discharged during the operating phase.

[0014] In addition, the method according to the invention is further developed such that the method is carried out on the control voltage of a phase locked loop. A specific additional voltage can thus also be produced here when required during a preparation phase, which, in an operating phase, is added to the variable control voltage of a phase locked loop. This results in the input voltage range of the oscillator of a phase locked loop, and hence also its pull-in range, being increased.

[0015] The method according to the invention is thus substantially suitable for those oscillators or phase locked loops which are not operating continuously.

[0016] The inventor furthermore proposes the further development of an electrical circuit arrangement for controlling a voltage controlled oscillator, with the oscillator having at least one control input for a variable control voltage, and at least one input for an operating voltage, such that a capacitor is provided in series with the control input of the oscillator, and may be precharged in a preparation phase. If an additional voltage, which is produced during the precharging, is now added to the control voltage, a higher control voltage can be made available to the oscillator at its control input.

[0017] In one preferred refinement of the electrical circuit arrangement according to the invention, at least one switching means is provided for precharging, with the capacitor being charged with the aid of a charging voltage depending on the position of the switching means. At least three, and possibly four, switching means are advantageously provided for precharging.

[0018] A further advantageous refinement of the electrical circuit arrangement according to the invention provides for at least one switching means to represent at least one transistor, at least one diode or at least one switch. The switching means may also be formed by combinations of transistors, diodes and switches.

[0019] One advantageous further development of the electrical circuit arrangement according to the invention is to provide a control apparatus for operating at least one switching means. This control apparatus interrupts the additional voltage of the capacitor by interrupting the charging voltage.

[0020] The control apparatus can furthermore be designed such that the charging time can be varied. For example, it may contain a timer which causes the control apparatus to interrupt the charging process after a predetermined charging time. By way of example, the capacitor may be charged via a resistor or a current source before the control apparatus opens a switch, and hence ends the charging process. Furthermore, the control apparatus may be in the form of a program part.

[0021] Since most voltage controlled oscillators are not operated in a free-running manner, but are part of a phase locked loop, the inventor proposes the further development of a phase locked loop having at least one detector which produces a variable control voltage, having a filter arrangement and having an oscillator, with the oscillator having at least one control input for the variable control voltage and at least one input for an operating voltage, such that the electrical circuit arrangement according to the invention as mentioned above is provided upstream of the control input of the oscillator. The filter arrangement is, for example, a loop filter, through which DC voltages can pass.

[0022] In one preferred embodiment of the development of the phase locked loop according to the invention, the detector has two complimentary arranged transistors, which each carry out the function of a switching means in a preparation phase. The transistors which are presents in the detector are thus also used for capacitor charging. This means that the capacitor can be both charged and discharged depending on the position of the transistors and a further switch. After the preparation phase, that is to say in the operating. phase, the transistors once again operate as phase detectors.

[0023] Further features of the invention can be found in the following description of a number of exemplary embodiments, with reference to the drawings, with an operating voltage of 2.8 V and a control voltage of 0.3 V to 2.2 V being assumed. It is also assumed that the control input of the oscillator has only a negligible current consumption (good approximation for varactor-controlled oscillators). In the figures:

[0024]FIG. 1a: shows a circuit arrangement according to the invention in a preparation phase with a capacitor being precharged;

[0025]FIG. 1b: shows a circuit arrangement according to the invention in an operating phase, after precharing of the capacitor;

[0026]FIG. 2a: shows a circuit arrangement according to the invention in the preparation phase, without the capacitor being precharged;

[0027]FIG. 2b: shows a circuit arrangement according to the invention in the operating phase, without the capacitor having been precharged;

[0028]FIGS. 3a,b: show a circuit arrangement according to the invention with an alternative switch arrangement in the preparation phase;

[0029]FIG. 4a: shows a circuit arrangement according to the invention in the preparation phase, for positive precharging of the capacitor;

[0030]FIG. 4b: shows a circuit arrangement according to the invention in the preparation phase, for negative precharging of the capacitor;

[0031]FIG. 5a: shows a circuit arrangement according to the invention with a variable bias voltage by means of a resistor;

[0032]FIG. 5b: shows a circuit arrangement according to the invention with a variable bias voltage by means of a current source;

[0033]FIG. 6: shows a known phase locked loop;

[0034]FIG. 7: shows a phase locked loop according to the invention with a capacitor.

[0035]FIG. 1a shows an electrical circuit arrangement according to the invention in a preparation phase, with the capacitor 2 being charged (precharging). The circuit arrangement comprises a circuit input 8, and input 7 for a charging voltage U3 as well as an oscillator 3, having a control input 4 for a control voltage U5, an RF output 5, a line to ground 15 and an input 6 for an operating voltage U1. The circuit arrangement also has three switches 1.1 to 1.3. The capacitor 2 is arranged between the circuit input 8 and the control input 4 of the oscillator 3.

[0036] A circuit-input-side line 20.1 runs between the circuit input 8 and the capacitor 2, and an oscillator-side line 20.2 runs between the capacitor 2 and the control input 4 of the oscillator 3. The switch 1.2 connects the input 7 to the line 20.2, the switch 1.1. connects the line 20.1 to ground, and the switch 1.3 connects the line 20.2 to ground.

[0037] In the preparation phase, the capacitor 2 is charged by the charging voltage U3 which is applied to the input 7, with the switches 1.1 and 1.2 being closed, and the switch 1.3 being open. The charging voltage U3 is in this case equal to the operating voltage U1 (2.8 V), represented by the dashed line between the inputs 7 and 6. At the end of the preparation phase, the capacitor 2 is at an additional voltage U2 (2.8 V), which is equal to the charging voltage U3.

[0038] It is self-evident that the charging voltage U3 need not be the same as the operating voltage U1, but may have a different value when, for example, a second operating voltage is provided.

[0039]FIG. 1b shows an electrical circuit arrangement according to the invention in an operating phase after the precharging of the capacitor 2. All the switches 1.1 to 1.3 are open, and a control voltage U4 (0.3 V to 2.2 V) is applied to the circuit input 8. The additional voltage U2 (2.8 V) of the capacitor 2 is added to the incoming control voltage U4, so that the control voltage US at the control input 4 of the oscillator 3 is now greater by the magnitude of the additional voltage U2 than the control voltage U4 at the circuit input 8. The control voltage US accordingly has values between 3.1 V and 5 V.

[0040] Since the oscillator 3 changes the frequency at its RF output 5 as a function of the control voltage U5, the pull-in range of the oscillator 3, that is to say the magnitude of its tunable frequency range, can be widened by increasing the control voltage U5.

[0041] If a low control voltage U5 is desired at the oscillator 3 rather than a high control voltage, then the procedure as illustrated in FIGS. 2a and 2 b is used.

[0042]FIG. 2a shows the circuit arrangement according to the invention in the preparation phase without the capacitor 2 being precharged. In this case, the grounded switches 1.1 and 1.3 are closed, and the switch 1.2 is opened, so that the capacitor 2 remains uncharged.

[0043]FIG. 2b shows the operating phase of the circuit arrangement, without the capacitor 2 having previously been charged. In this case, the control voltage U5 at the control input 4 of the oscillator 3 has the same value as the control voltage U4 at the circuit input 8, namely 0.3 V to 2.2 V.

[0044] If the circuit from FIGS. 1a, 1 b, 2 a and 2 b is operated using the control voltage U4 from 0.3 V to 2.2 V and using an operating voltage U1 and a charging voltage U3 of 2.8 V in each case, then this results in the control voltage U5 of 0.3 V to 2.2 V at the control input 4 of the oscillator 3, without the capacitor 2 having been precharged, and 3.1 V to 5 V with precharging. This results in a gap of from 2.2 V to 3.1 V, which is not covered.

[0045] This gap can be overcome by choosing a charging voltage U3 which is less than or equal to 1.9 V. However, since it may be undesirable to provide a further operating voltage U1, FIG. 1a is developed as is shown in FIGS. 5a and 5 b.

[0046]FIGS. 3a and 3 b show alternative exemplary embodiments of the circuit arrangement according to the invention, which may achieve the same purpose as the circuit arrangements in the previous figures. FIGS. 3a and 3 b show the switch positions in the preparation phase.

[0047] In FIG. 3a, the switch 1.1 connects the circuit-input-side line 20.1 to ground, the switch 1.2 connects the input 7 to the oscillator-side line 20.2, and the switch 1.3 connects the line 20.1 to the line 21.1, which runs between the switch 1.2 and the input 7. In order to charge the capacitor 2 to an additional voltage U2, the switches 1.1 and 1.2 are closed and the switch 1.3 is open.

[0048] In a subsequent operating phase all the switches 1.1 to 1.3 are open, so that the additional voltage U2 across the capacitor 2 is added to the control voltage U4.

[0049] In FIG. 3b, the switch 1.1 also connects the line 20.1 to ground, and the switch 1.2 connects the input 7 to the line 20.2. In this case, the switch 1.3 now connects the line 20.1 to the line 21.2, which runs between the switch 1.2 and the line 20.2. As in FIG. 3a, the switches 1.1 and 1.2 are closed for charging the capacitor 2, and the switch 1.3 is opened. If the capacitor 2 is to be discharged, this is achieved by opening the switch 1.2 and closing the switch 1.3.

[0050]FIGS. 4a and 4 b show two exemplary embodiments of a so-called full bridge with the four switches 1.1 to 1.4.

[0051] In FIG. 4a, the switch 1.1 connects the line 20.1 to ground, the switch 1.3 (which is arranged in parallel with it) connects the line 20.2 to ground, and the switch 1.2 connects the input 7 to the line 20.2. The connection between the line 20.1 and the line 21.1 is produced via the switch 1.4. When the switches 1.3 and 1.4 are open and the switches 1.1 and 1.2 are closed, the capacitor 2 is charged to a positive additional voltage U2, as is shown in FIG. 4a.

[0052] The full bridge in FIG. 4b allows the capacitor to be charged to negative additional voltages u2. The arrangement of the switches 1.1 to 1.4 is analogous to the switch arrangement shown in FIG. 4a, but with the switches 1.2 and 1.1 now being open, and the switches 1.3 and 1.4 being closed.

[0053] All the switches 1.1 to 1.4 are open, both in FIG. 4a and in FIG. 4b, during an operating phase that follows the preparation phase.

[0054]FIG. 5a shows one preferred embodiment of the circuit arrangement according to the invention, in which the additional voltage U2 across the capacitor 2 can be varied.

[0055] In contrast to FIG. 1a, there is a resistor 12 between the input 7 for the charging voltage U3 and the switch 1.2. The circuit arrangement also has a controller 13, which can open and close the switch 1.2, represented by the dashed line. The capacitor 2 is now charged on the basis of the charging voltage U3, which is applied to the input 7, via the resistor 12. As in FIG. 1a, the switches 1.2 and 1.1 are closed, and the switch 1.3 is open. After a predetermined charging time, the controller 13 opens the switch 1.2 and thus ends the process of charging the capacitor 2. During the subsequent operating phase, all the switches 1.1 to 1.3 are open.

[0056] This preferred embodiment allows additional voltages U2 to be produced across the capacitor 2 which are between 0 V and the charging voltage U3 or the operating voltage U1. When additional voltages U2 between 0.1 V and 0.9 V are produced, this makes it possible to cover the gap, which occurs in FIGS. 1a to 2 b, from 2.2 V to 3.1 V in the control voltage U5 at the input 4 of the oscillator 3.

[0057]FIG. 5b shows another circuit arrangement according to the invention, with the resistor 12 from FIG. 5a being replaced by a current source.

[0058]FIG. 6 shows the design of a conventional phase locked loop. In a phase locked loop, the output frequency of the oscillator 3 has a rigid phase relationship with a reference frequency by virtue of a filter arrangement 11 (for example a loop filter), a phase/frequency detector 10, an oscillator 3, which has a control input 4, an input 6 for the operating voltage U1 and an RF output 5, and a frequency divider (not illustrated here).

[0059] The detector 10 produces a control voltage U4, which passes through the filter arrangement 11 and reaches the control input 4 of the oscillator 3. Depending on the magnitude of the control voltage U4, an output frequency is generated, which is supplied to the detector 10 once again, via the frequency divider. This detector 10 then compares the output frequency with the reference frequency, and regulates the control voltage U4 appropriately.

[0060] In this case, a phase detector which has two transistors 19.X (charge pump transistors) is shown as the detector 10. The output of this detector 10 has current source characteristics, with the transistors 19.X simulating the current sources. Other variants for a detector 10 would be, for example, a simple analogue mixer, an EXOR gate or a ring mixer.

[0061]FIG. 7 shows one preferred exemplary embodiment of the phase locked loop according to the invention, which represents a development of the conventional phase locked loop shown in FIG. 6. In this case, a capacitor 2 is arranged between the filter arrangement 11 and the control input 4 of the oscillator 3. The capacitor 2 may also be arranged upstream of the filter arrangement 11, although, in this variant, relatively large capacitance values are required for the capacitor 2.

[0062] Furthermore, a switch 1.2 connects the input 7 for the charging voltage U3 to the oscillator-side line 20.2 between the capacitor 2 and the oscillator 3. In this case, the filter arrangement 11 is in the form of a loop filter, through which DC voltages can normally pass.

[0063] A passive loop filter (capacitor) is preferably used as the filter arrangement 11, which has constant current sources which charge or discharge the filter 11. In order to achieve better interference suppression, a tuning voltage produced in this way is also passed via an RC low-pass filter. The loop filter 11 may thus have a series resistance (DC resistance) which does not interfere with the operation of the circuit in FIG. 7, however, provided it is sufficiently small. This means that the DC resistance should be so small that it is possible to fully charge the capacitor 2 during the preparation phase, despite the DC resistance.

[0064] The output 9 of the phase/frequency detector 10 has two complimentary arranged transistors 19.1 and 19.2 (charge pump transistors), that is to say, by way of example, two MOSFETs. Each of these individual MOSFETs may, if desired, be opened or closed as required. If they are suitably controlled, these transistors 19.X may thus, in the preparation phase, carry out the function of the switch 1.1 from FIGS. 1a and 2 a, and the function of the switches 1.1 and 1.3 from FIGS. 3a and 3 b.

[0065] In order to charge the capacitor 2 to the additional voltage U2, the switch 1.2 and the transistor 19.1 are closed in the preparation phase. In contrast, the switch 19.2 is open.

[0066] In the operating phase, the switch 1.2 is open, the transistors 19.1 and 19.2 operate as normal phase detectors, and the additional voltage U2 is added to the control voltage U4 emitted from the detector 10. The control voltage U5 is now applied to the input 4 of the oscillator 3, and is composed of the additional voltage U2 and the control voltage U4. Thus, in this operating phase, the transistors 19.1 and 19.2 once again operate as normal, as phase detectors.

[0067] It is self-evident that other phase detectors may also be used instead of a detector 10 having transistors 19.X, such as a mixer which has no output transistors 19.X, for example. According to the invention, one of the circuit arrangements according to the invention as described above from FIGS. 1a to 5 b may be located between the phase detector and the oscillator 3, and can be used to increase the control voltage U5 at the control input 4 of the oscillator 3. In this case, an active loop filter may now also be used as the filter arrangement 11.

[0068] It is self-evident that the abovementioned features of the invention can be used not only in the respectively stated combination but also in other combinations or on their own, without departing from the scope of the invention.

[0069] Overall, the invention provides a method which allows the pull-in range of an oscillator to be widened in a simple and disturbance-free manner. Furthermore, an electrical circuit arrangement and a phase locked loop are proposed, in order to carry out the method.

[0070] List of Reference Symbols List of reference symbols 1.X Switching means 2 Capacitor 3 Oscillator 4 Control input 5 RF output 6 Input for the operating voltage U1 7 Input for the charging voltage U3 8 Circuit input 9 Output of the detector 10 10 Detector 11 Filter arrangement 12 Resistor 13 Control apparatus 14 Current source 15 Line to ground 19.X Transistor 20.1 Circuit-input-side line 20.2 Oscillator-side line 21.1 Line between switch 1.2 and input 7 21.2 Line between switch 1.2 and line 20.2 U1 Operating voltage U2 Additional voltage U3 Charging voltage U4 Control voltage U5 Control voltage 

1. Method for operating a voltage controlled oscillator (3), in particular in a portable communications appliance, with the oscillator (3) being supplied with a variable control voltage (U4) which is taken from an operating voltage (U1), preferably from a constant voltage source, characterized in that the variable control voltage (U4) is supplied via a capacitor (2) to the oscillator (3), and a specific additional voltage (U2) is added to the variable control voltage (U4) when required in an operating phase, with the specific additional voltage (U2) being applied to the capacitor (2) in a preparation phase.
 2. The method as claimed in the preceding claim 1, characterized in that the capacitor (2) is charged during the preparation phase with the aid of a charging voltage (U3) which is applied to the capacitor (2) and may be equal to the operating voltage (U1).
 3. The method as claimed in the preceding claim 2, characterized in that the specific additional voltage (U2) of the capacitor (2) is controlled by interrupting the charging voltage (U3) for the capacitor (2).
 4. The method as claimed in the preceding claims 1 to 3, characterized in that the additional voltage (U2) is formed in the opposite sense to the charging voltage (U3).
 5. The method as claimed in the preceding claims 1 to 4, characterized in that the capacitor (2) is discharged during the preparation phase.
 6. The method as claimed in the preceding claims 1 to 5, characterized in that the capacitor (2) is neither charged nor discharged during the operating phase.
 7. The method as claimed in the preceding claims 1 to 6, characterized in that the method is carried out on the control voltage of a phased locked loop.
 8. An electrical circuit arrangement for controlling a voltage controlled oscillator (3), with the oscillator (3) having at least one control input (4) for a variable control voltage (U4) and at least one input (6) for an operating voltage (U1), characterized in that the capacitor (2) is provided in series with the control input (4) of the oscillator (3) and may be precharged in a preparation phase.
 9. The electrical circuit arrangement as claimed in the preceding claim 8, characterized in that at least one switching means (1.X) is provided for precharging, with the capacitor (2) being charged with the aid of a charging voltage (U3) depending on the position of the at least one switching means (1.X).
 10. The electrical circuit arrangement as claimed in the preceding claim 9, characterized in that at least three, and possibly four, switching means (1,X) are provided for precharging.
 11. The electrical circuit arrangement as claimed in one of the preceding claims 9 and 10, characterized in that at least one switching means (1.X) is at least one transistor and/or at least one diode and/or at least one switch.
 12. The electrical circuit arrangement as claimed in one of the preceding claims 9 to 11, characterized in that a control apparatus (13) is provided for operating at least one switching means (1.X).
 13. The electrical circuit arrangement as claimed in the preceding claim 12, characterized in that the control apparatus (13) is designed such that the charging time can be varied.
 14. A phase locked loop having at least one detector (10) which produces a variable control voltage (U4), having a filter arrangement (11) and having an oscillator (3), with the oscillator (3) having at least one control input (4) for the variable control voltage (U4) and at least one input (6) for an operating voltage (U1), characterized in that an electrical circuit arrangement as claimed in one of the features of the preceding claims 8 to 13 is provided upstream of the control input (4).
 15. A phased locked loop as claimed in the preceding claim 14, characterized in that the detector (10) has two complimentary arranged transistors, which act as two switching means (1.X) in a preparation phase. 